Production of a GaN bulk crystal substrate and a semiconductor device formed thereon

ABSTRACT

A crystal has a diameter of 1 cm or more and shows a strongest peak in cathode luminescent spectrum at a wavelength of 360 nm in correspondence to a band edge.

BACKGROUND OF THE INVENTION

The present invention generally relates to semiconductor devices andmore particularly to a semiconductor device having a GaN bulk crystalsubstrate.

GaN is a III-V compound semiconductor material having a large bandgap ofblue to ultraviolet wavelength energy. Thus, intensive investigationsare being made with regard to development of optical semiconductordevices having a GaN active layer for use particularly in opticalinformation storage devices including a digital video data recorder(DVD). By using such a light emitting semiconductor device producingblue to ultraviolet wavelength optical radiation for the optical source,it is possible to increase the recording density of optical informationstorage devices.

Conventionally, a laser diode or light-emitting diode having a GaNactive layer has been constructed on a sapphire substrate in view of theabsence of technology of forming a GaN bulk crystal substrate.

FIG. 1 shows the construction of a conventional GaN laser diodeaccording to Nakamura, S., et al., Jpn. J. Appl. Phys. vol. 36 (1997)pp. L1568-L1571, Part 2, No. 12A, 1 Dec. 1997, constructed on a sapphiresubstrate 1.

Referring to FIG. 1, the sapphire substrate 1 has a (0001) principalsurface covered by a low-temperature GaN buffer layer 2, and includes aGaN buffer layer 3 of n-type grown further on the buffer layer 2. TheGaN buffer layer 3 includes a lower layer part 3 a and an upper layerpart 3 b both of n-type, with an intervening SiO₂ mask pattern 4provided such that the SiO₂ mask pattern 4 is embedded between the lowerlayer part 3 a and the upper layer part 3 b. More specifically, the SiO₂mask pattern 4 is formed on the lower GaN buffer layer part 3 a,followed by a patterning process thereof to form an opening 4A in theSiO₂ mask pattern 4.

After the formation of the SiO₂ mask pattern 4, the upper GaN layer part3 b is formed by an epitaxial lateral overgrowth (ELO) process in whichthe layer 3 b is grown laterally on the SiO₂ mask 4. Thereby, desiredepitaxy is achieved with regard to the lower GaN layer part 3 a at theopening 4A in the SiO₂ mask pattern 4. By growing the GaN layer part 3 bas such, it is possible to prevent the defects, which are formed in theGaN layer part 3 a due to the large lattice misfit between GaN andsapphire, from penetrating into the upper GaN layer part 3 b.

On the upper GaN layer 3 b, a strained super-lattice structure 5 havingan n-type Al_(0.14)Ga_(0.86)N/GaN modulation doped structure is formed,with an intervening InGaN layer 5A of the n-type having a compositionIn_(0.1)Ga_(0.9)N interposed between the upper GaN layer part 3 b andthe strained superlattice structure 5. By providing the strainedsuperlattice structure 5 as such, dislocations that are originated atthe surface of the sapphire substrate 1 and extending in the upwarddirection are intercepted and trapped.

On the strained superlattice structure 5, a lower cladding layer 6 ofn-type GaN is formed, and an active layer 7 having an MQW structure ofIn_(0.01)Ga_(0.98)N/In_(0.15)Ga_(0.85)N is formed on the cladding layer6. Further, an upper cladding layer 8 of p-type GaN is formed on theactive layer 7, with an intervening electron blocking layer 7A of p-typeAlGaN having a composition of Al_(0.2)Ga_(0.8)N interposed between theactive layer 7 and the upper cladding layer 8.

On the upper cladding layer 8, another strained superlattice structure 9of a p-type Al_(0.14)Ga_(0.86)N/GaN modulation doped structure is formedsuch that the superlattice structure 9 is covered by a p-type GaN caplayer 10. Further, a p-type electrode 11 is formed in contact with thecap layer 10 and an n-type electrode 12 is formed in contact with then-type GaN buffer layer 3 b.

It is reported that the laser diode of FIG. 1 oscillates successfullywith a practical lifetime, indicating that the defect density in theactive layer 7 is reduced successfully.

On the other hand, the laser diode of FIG. 1 cannot eliminate thedefects completely, and there remain substantial defects particularly incorrespondence to the part on the SiO₂ mask 4 as represented in FIG. 2.See Nakamura S. et al., op cit. It should be noted that such defectsformed on the SiO₂ mask 4 easily penetrate through the strainedsuperlattice structure 5 and the lower cladding layer 6 and reach theactive layer 7.

In view of the foregoing concentration of the defects in the centralpart of the SiO₂ mask pattern 4, the laser diode of FIG. 1 uses the partof the semiconductor epitaxial structure located on the opening 4A ofthe SiO₂ mask 4, by forming a mesa structure M in correspondence to theopening 4A. However, the defect-free region formed on the opening 4A hasa lateral size of only several microns, and thus, it is difficult toconstruct a high-power laser diode based on the construction of FIG. 1.When the laser diode of FIG. 1 is driven at a high power, the area ofoptical emission in the active region extends inevitably across thedefects, and the laser diode is damaged as a result of opticalabsorption caused by the defects. Further, the laser diode of FIG. 1having such a construction has other various drawbacks associated withthe defects in the semiconductor epitaxial layers, such as largethreshold current, limited lifetime, and the like. Further, the laserdiode of FIG. 1 has a drawback, in view of the fact that the sapphiresubstrate is an insulating substrate, in that it is not possible toprovide an electrode on the substrate. As represented in FIG. 1, it isnecessary to expose the top surface of the n-type GaN buffer layer 3 byan etching process in order to provide the n-type electrode 12, whilesuch an etching process complicates the fabrication process of the laserdiode. In addition, the increased distance between the active layer 7and the n-type electrode 12 causes the problem of increased resistanceof the current path, while such an increased resistance of the currentpath deteriorates the high-speed response of the laser diode.

Further, the conventional laser diode of FIG. 1 suffers from the problemof poor quality of mirror surfaces defining the optical cavity. Due tothe fact that the sapphire single crystal constituting the substrate 1belongs to hexagonal crystal system, formation of the optical cavitycannot be achieved by a simple cleaving process. It has been thereforenecessary to form the mirror surfaces, when fabricating the laser diodeof FIG. 1 by conducting a dry etching process, while the mirror surfacethus formed by a dry etching process has a poor quality.

Because of the foregoing reasons, as well as because of other variousreasons not mentioned here, it is desired to form the substrate of theGaN laser diode by a bulk crystal GaN and form the laser diode directlyon the GaN bulk crystal substrate.

With regard to the art of growing a bulk crystal GaN, there is asuccessful attempt reported by Porowski (Porowski, S., J. Crystal Growth189/190 (1998) pp. 153-158, in which a GaN bulk crystal is synthesizedfrom a Ga melt under an elevated temperature of 1400-1700° C. and anelevated N₂ pressure of 12-20 kbar (1.2-2 GPa). This process, however,can only provide an extremely small crystal in the order of 1 cm indiameter at best. Further the process of Porowski requires a speciallybuilt pressure-resistant apparatus and a long time is needed for loadingor unloading a source material, or increasing or decreasing the pressureand temperature. Thus, the process of this prior art would not be arealistic solution for mass-production of a GaN bulk crystal substrate.It should be noted that the reaction vessel of Porowski has to withstandthe foregoing extremely high pressure, which is rarely encountered inindustrial process, under the temperature exceeding 1400° C.

Further, there is a known process of growing a GaN bulk crystal withoutusing an extremely high pressure environment for growing a GaN bulkcrystal as reported by Yamane, H., et al., Chem. Mater. 1997, 9,413-416. More specifically, the process of Yamane et al. successfullyavoids the use of the extremely high-pressure used in Porowski, byconducting the growth of the GaN bulk crystal from a Ga melt in thepresence of a Na flux.

According to the process of Yamane, a metallic Ga source and a NaN₃(sodium azide) flux are confined in a pressure-resistance reactionvessel of stainless steel together with a N₂ atmosphere, and thereaction vessel is heated to a temperature of 600-800° C. and held for aduration of 24-100 hours. As a result of the heating, the pressureinside the reaction vessel is elevated to the order of 100 kg/cm² (about10 MPa), which is substantially lower than the pressure used byPorowski. As a result of the reaction, GaN crystals are precipitatedfrom the melt of a Na—Ga system. In view of the relatively low pressureand low temperature needed for the reaction, the process of Yamane etal. is much easier to implement.

On the other hand, the process of Yamane relies upon the initiallyconfined N₂ molecules in the atmosphere and the N atoms contained in theNaN₃ flux for the source of N. Thus, when the reaction proceeds, the N₂molecules in the atmosphere or the N atoms in the Na—Ga melt aredepleted with the precipitation of the GaN crystal, and there appears alimitation in growing a large bulk crystal of GaN. The GaN crystalsobtained by the process of Yamane et al. typically have a size of 1 mmor less in diameter. Thus, the process of Yamane et al. op cit., whilebeing successful in forming a GaN bulk crystal at a relatively lowpressure and temperature, cannot be used for a mass production of a GaNsubstrate in the industrial base.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful GaN semiconductor device having a bulk crystalsubstrate wherein the foregoing problems are eliminated.

Another and more specific object of the present invention is to providea process of making a bulk crystal substrate of a GaN single crystal.

Another object of the present invention is to provide a process offabricating a GaN semiconductor device having a bulk crystal substrateof a GaN single crystal.

Another object of the present invention is to provide a bulk crystalsubstrate of a single crystal GaN.

Another object of the present invention is to provide an opticalsemiconductor device having a bulk crystal substrate of a GaN singlecrystal.

Another object of the present invention is to provide an electron devicehaving a bulk crystal substrate of a GaN single crystal.

Another object of the present invention is to provide an apparatus formaking a bulk crystal substrate of a GaN single crystal.

According to the present invention, a high-quality GaN bulk crystalsubstrate is obtained with a process suitable for mass-production, bycontinuously supplying N so as to compensate for the depletion of Noccurring in the system in which precipitation of a GaN single crystaltakes place. By using the GaN bulk crystal substrate thus obtained, itis possible to fabricate an optical semiconductor device that producesan optical radiation of blue to ultraviolet wavelength with a largeoptical power. Further, the GaN bulk crystal substrate can be used as asubstrate of an electron device such as HEMT.

Other objects and further features of the present invention will becomeapparent from the following detailed description when read inconjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the construction of a conventional laserdiode constructed on a sapphire substrate;

FIG. 2 is a diagram showing the problem associated with the laser diodeof FIG. 1;

FIG. 3 is a diagram showing the construction of a growth apparatus usedin a first embodiment of the present invention for growing a GaN bulkcrystal;

FIGS. 4A and 4B are diagrams showing a part of the apparatus of FIG. 3in detail;

FIG. 5 is a diagram showing a cathode luminescent spectrum of a GaN bulkcrystal obtained in the first embodiment;

FIG. 6 is a diagram showing a control of GaN composition in the growthapparatus of FIG. 3;

FIG. 7 is a diagram showing the construction of a growth apparatus usedin a second embodiment of the present invention for growing a GaN bulkcrystal;

FIG. 8 is a diagram showing the construction of a growth apparatus usedin a third embodiment of the present invention for growing a GaN bulkcrystal;

FIG. 9 is a diagram showing the construction of a growth apparatus usedin a fourth embodiment of the present invention for growing a GaN bulkcrystal;

FIG. 10 is a diagram showing the construction of a growth apparatus usedin a fifth embodiment of the present invention for growing a GaN bulkcrystal;

FIG. 11 is a diagram showing the construction of a growth apparatus usedin a sixth embodiment of the present invention for growing a GaN bulkcrystal;

FIG. 12 is a diagram showing the construction of a growth apparatus usedin a seventh embodiment of the present invention for growing a GaN bulkcrystal;

FIG. 13 is a diagram showing the construction of a seed crystal used inthe growth apparatus of FIG. 12;

FIG. 14 is a diagram showing the construction of a growth apparatus usedin an eighth embodiment of the present invention for growing a GaN bulkcrystal;

FIGS. 15A and 15B are diagrams showing a part of the growth apparatus ofFIG. 14;

FIG. 16 is a diagram showing the growth apparatus of FIG. 14 in thestate in which a growth of the GaN bulk crystal has been made;

FIG. 17 is a diagram showing the construction of a growth apparatus usedin a ninth embodiment of the present invention for growing a GaN bulkcrystal;

FIG. 18 is a diagram showing the construction of a growth apparatus usedin a tenth embodiment of the present invention for growing a GaN bulkcrystal;

FIG. 19 is a diagram showing X-ray diffraction data obtained for a GaNbulk crystal according to an eleventh embodiment of the presentinvention;

FIG. 20 is a diagram showing the construction of a laser diode having aGaN bulk crystal substrate according to a twelfth embodiment of thepresent invention; and

FIG. 21 is a diagram showing the construction of a HEMT having a GaNbulk crystal substrate according to a thirteenth embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

FIG. 3 shows the construction of a growth apparatus 100 used in a firstembodiment of the present invention for growing a GaN bulk crystal.

Referring to FIG. 3, the growth apparatus 100 includes apressure-resistant reaction vessel 101 typically of a stainless steelhaving an inner diameter of about 75 mm and a length of about 300 mm andaccommodates therein a crucible 102 of Nb or BN. As will be explainedlater, the crucible 102 is loaded with a starting material of metallicGa and a NaN₃ flux and is confined in the reaction vessel 101 togetherwith an N₂ atmosphere 107. Further, the reaction vessel 101 is suppliedwith N₂ or a gaseous compound of N from an external source via aregulator valve 109 and an inlet 108. The reaction vessel 101 thusloaded with the starting material in the crucible 102 is heated byenergizing heaters 110 and 111 to a temperature of 650-850° C., and thepressure inside the reaction vessel is regulated to a moderate value ofabout 5 MPa by controlling the valve 109. By holding the temperature andthe pressure, a precipitation of GaN bulk crystal takes place from aNa—Ga melt, which is formed in the crucible 102 as a result of themelting of the starting material.

FIG. 4A shows the loading of the starting material in the crucible 102,while FIG. 4B shows the state in which the source material has caused amelting.

Referring to FIG. 4A, a high-purity metallic Ga and a high-puritymetallic Na are weighed carefully and loaded into the crucible 102,wherein the foregoing process of weighing and loading are conducted inthe N₂ atmosphere. It is also possible to use high-purity NaN₃ in placeof high-purity metallic Na source.

In the state of FIG. 4B, on the other hand, there appears a melt 102A ofthe Na—Ga system in the crucible 102 and crystallization of GaN takesplace from various parts of the melt 102A including a free surface ofthe melt and a sidewall or bottom wall of the crucible 102. There, itwas observed that a large single crystal 102B of GaN grows on the meltfree surface contacting with the atmosphere and fine needle-like GaNcrystals 102C grow on the sidewall or bottom wall of the crucible 102.

With the growth of the GaN crystals, particularly with the growth of theGaN single crystal 102B, N in the atmosphere is consumed and thepressure inside the reaction vessel gradually falls as a result ofdepletion of N in the atmosphere. Thus, in the present embodiment, thedepletion of N in the atmosphere 107 is compensated for by replenishingN₂ or a compound of N such as NH₃ from an external source. Thereby, thegrowth of the GaN single crystal 102B continues at the melt free surfaceand a large GaN single crystal suitable for use in an opticalsemiconductor device such as a laser diode or light-emitting diode as aGaN bulk crystal substrate is obtained. The construction of FIG. 3 caneasily produce the GaN single crystal 102B with a thickness of 100 μm ormore. The GaN single crystal 102B thus formed at the temperature of650-850° C. has a hexagonal crystal symmetry.

FIG. 5 shows the cathode luminescent spectrum of the GaN single crystal102B thus obtained in comparison with the cathode luminescent spectrumof a GaN thick film grown on a sapphire substrate or an SiC substrate.

Referring to FIG. 5, it can be seen that the GaN crystal 102B of thepresent embodiment shows a distinct and strong peak corresponding to theband edge of GaN at the wavelength of about 360 nm. Further, it can beseen that no other peak exists in the GaN single crystal 102B of thepresent embodiment. The result of FIG. 5 indicates that the GaN crystal102B thus formed has a defect density of less than 10²-10³ cm⁻². Thus,the GaN single crystal 102B is suitable for use as a bulk GaN substrateof various optical semiconductor devices including a laser diode and alight-emitting diode as noted already. Hereinafter, the GaN singlecrystal 102B will be called a GaN bulk crystal in view of application toa GaN bulk crystal substrate.

Contrary to the present embodiment, the GaN thick film formed on thesapphire substrate or formed on the SiC substrate shows a remarkablepeak at the wavelength of about 600 nm corresponding to deep impuritylevels. This clearly indicates that the GaN thick film thus formed on asapphire substrate or an SiC substrate contains a substantial amount ofdefects. Associated with the high level of defects, it can be seen thatthe peak strength for the band edge is substantially smaller than thecase of the GaN bulk crystal 102B of the present embodiment.

In the growth process of FIG. 4B, it should be noted that there appearsalso an intermetallic compound 102D of GaNa along the sidewall andbottom surface of the crucible 102 indicated in FIG. 4B by a brokenline. Thus, the region represented in FIG. 4B by the broken line in factincludes the fine GaN crystals 102C and the GaNa intermetallic compound102D in the form of a mixture. The GaN fine crystals 102C or the GaNaintermetallic compound 102D thus formed releases Ga into the melt 102A,and the Ga atoms thus released contribute to the growth of the GaN bulkcrystal 102B when transported to the melt surface.

Thus, by continuously replenishing N₂ or NH₃, the growth process of theGaN bulk crystal 102B continues until Ga in the melt 102A is used up.

FIG. 6 shows the control of the N₂ pressure in the atmosphere 107 withthe growth of the GaN bulk crystal 102B from the melt 102A.

Referring to FIG. 6, it can be seen that the N₂ pressure a necessary formaintaining the stoichiometric composition for the GaN bulk crystal 102Bchanges depending on the Ga content in the melt 102A represented in thehorizontal axis. When the N₂ pressure in the atmosphere 107 is fixed(a₁=a₂), it is not possible to maintain the stoichiometric compositionfor the GaN bulk crystal 102B. Thus, the present invention changes theN₂ pressure a in the atmosphere 107 with the progress of growth of theGaN bulk crystal 102B as represented as a₁≠a₂

Second Embodiment

FIG. 7 shows the construction of a growth apparatus 200 according to asecond embodiment of the present invention, wherein those partscorresponding to the parts described previously are designated by thesame reference numerals and the description there of will be omitted.

Referring to FIG. 7, the present embodiment uses heaters 111A and 111Bin place of the heater 111 and induces a temperature gradient in themelt 102A for facilitating transport of Ga from the GaN fine crystals102C or the GaNa intermetallic compound 102D to the melt surface.

More specifically, the heater 111B is provided in correspondence to thebottom part of the crucible 102 and controls, together with the heart111A, the melt temperature at the bottom part of the crucible 102 lowerthan the melt surface. As a result of energization of the heaters 111Aand 111B, a temperature gradient shown in FIG. 7 is induced.

Due to the increased temperature at the bottom part of the crucible 102,undesirable precipitation of GaN crystals on bottom surface of thecrucible 102 is minimized, and the growth of the GaN bulk crystal 102Bon the melt surface is promoted substantially. When a GaN fine crystal102C is formed, such a GaN fine crystal 102C is immediately dissolvedinto the melt 102A and no substantial deposition occurs on the bottompart of the crucible 102. Further, the intermetallic compound of GaNa,formed at a temperature lower than about 530° C., acts also as thesource of Ga and Na in the melt 102A.

Similarly to the first embodiment, the GaN bulk crystal 102B formedaccording to the present embodiment has a defect density in the order of10²-10³ cm⁻² or less. Thus, the GaN bulk crystal 102B is suitable for abulk GaN substrate of various optical semiconductor devices including alaser diode and a light-emitting diode.

Third Embodiment

FIG. 8 shows the construction of a growth apparatus 300 according to athird embodiment of the present invention, wherein those partscorresponding to the parts described previously are designated by thesame reference numerals and the description thereof will be omitted.

Referring to FIG. 8, the present embodiment is a modification of theembodiment of FIG. 7 and uses the heaters 110 and 111, described withreference to the growth apparatus 100 for inducing the desiredtemperature gradient. As other aspects of the present embodiment aresubstantially the same as those of the previous embodiment, furtherdescription will be omitted.

Fourth Embodiment

FIG. 9 shows the construction of a growth apparatus 400 according to afourth embodiment of the present invention, wherein those partscorresponding to the parts described previously are designated by thesame reference numerals and the description thereof will be omitted.

Referring to FIG. 9, the growth apparatus 400 has a construction similarto that of FIG. 3, except that there is provided a container 103 holdinga metallic Ga source 104 inside the reaction vessel 101. The container103 is provided at a first end of a tube 103A extending outside of thereaction vessel 101, and there is provided a pressure regulator 106 at asecond, opposite end of the tube 103. The pressure regulator 106 issupplied with a pressurized N₂ gas from an external source and causes amolten Ga, formed in the container 103 as a result of heating, to dripto the Na—Ga melt 102A in the crucible 102 via a hole 105 formed at abottom part of the container 103.

According to the construction of FIG. 9, depletion of Ga in the melt102A is replenished from the Ga source 104 and a thickness of 300 μm ormore is obtained for the GaN bulk crystal 102B as a result of thecontinuous crystal growth.

Similarly to the previous embodiments, the GaN bulk crystal 102B formedaccording to the present embodiment has a defect density of 10²-10³ cm⁻²or less. Thus, the GaN bulk crystal 102B of the present embodiment issuitable for use as a bulk GaN substrate of various opticalsemiconductor devices including a laser diode and a light-emittingdiode.

Fifth Embodiment

FIG. 10 shows the construction of a growth apparatus 500 according to afifth embodiment of the present invention, wherein those partscorresponding to the parts described previously are designated by thesame reference numerals and the description thereof will be omitted.

Referring to FIG. 10, the growth apparatus 500 has a constructionsimilar to that of the growth apparatus of FIG. 9, except that there isprovided an outer pressure vessel 112 outside the reaction vessel 101,and the space between the reaction vessel 101 and the outer pressurevessel 112 is filled with a pressurized gas such as N₂, which isintroduced via a regulator 114 and an inlet 113.

By providing the pressure vessel 112 outside the reaction vessel 101,the pressurized reaction vessel 101 is supported from outside and thedesign of the reaction vessel 101 becomes substantially easier. Asrepresented in FIG. 10, there is provided a thermal insulator 115between the heater 110 or 111 and the outer pressure vessel 112 and thetemperature rise of the pressure vessel 112 is avoided. Thereby, thepressure vessel 112 maintains a large mechanical strength even when theinner, reaction vessel 101 is heated to the temperature exceeding 600 or700° C. In order to avoid the decrease of mechanical strength, it ispossible to provide a water cooling system (not shown) on the outerpressure vessel 112.

The outer pressure vessel 112 can be provided also to the growingapparatuses 100-300 explained before as well as to the growingapparatuses to be described hereinafter.

As other features of the present embodiment are substantially the sameas those of the previous embodiments, further description thereof willbe omitted.

Sixth Embodiment

FIG. 11 shows the construction of a growing apparatus 600 according to asixth embodiment of the present invention, wherein those partscorresponding to the parts described previously are designated by thesame reference numerals and the description thereof will be omitted.

Referring to FIG. 11, the growing apparatus 600 has a constructionsimilar to that of the growing apparatus 100 of FIG. 3, except thatthere is provided a holder 601 holding a Ga—Na melt outside the reactionvessel 101 and the Ga—Na melt in the holder 601 is supplied into thereaction vessel 101 and to the melt 102A in the crucible 102 via a tube601A penetrating through a wall of the reaction vessel 101, in responseto a pressurization of the holder 601 by a pressurized gas such as an N₂gas supplied via a line 602.

According to the present embodiment, the depletion of Ga in the melt102A is replenished together with the Na flux, and the growth of the GaNbulk crystal 102B at the free surface of the melt 102A is conductedcontinuously. It should be noted that depletion of N in the system isalso replenished by the external N source similarly to the previousembodiments. As a result, a high-quality GaN bulk crystal suitable foruse as a substrate of various optical semiconductor devices is obtainedwith a thickness well exceeding 100 μm, generally about 300 μm or more.

Seventh Embodiment

FIG. 12 shows the construction of a growth apparatus 700 according to aseventh embodiment of the present invention, wherein those partscorresponding to the parts described previously are designated by thesame reference numerals and the description thereof will be omitted.

Referring to FIG. 12, the growing apparatus 700 has a constructionsimilar to that of the growing apparatus 600 of the previous embodiment,except that there is provided a rod 702 carrying a seed crystal 701 at atip end thereof in contact with the free surface of the melt 102A in thecrucible 102. Further there is provided a motor 703 for pulling up therod 702, and there occurs a continuous growth of the GaN bulk crystal102B at the melt surface with the pulling up of the rod 702. Thereby, aningot of a GaN bulk crystal is obtained.

By slicing the GaN bulk crystal ingot thus obtained, it is possible tomass produce the GaN bulk crystal substrate for use in various opticalsemiconductor devices including a laser diode and a light-emittingdiode.

FIG. 13 shows an example of the seed crystal 701 provided at the tip endof the rod 702.

Referring to FIG. 13, the seed crystal 702 is formed to have a slabshape with a width w and a thickness d corresponding to the width andthickness of the GaN substrate to be formed. Thus, by pulling up the rod702 straight in the upward direction, a slab-shaped GaN bulk crystal isgrown continuously. Thus, by merely polishing the surface of the GaNbulk crystal slab, followed by a cleaving process, it is possible tomass-produce the GaN bulk crystal substrate having a quality suitablefor use in various optical semiconductor devices including a laser diodeand a light-emitting diode.

As other features of the present embodiment are more or less the same asthose of the previous embodiments, further description thereof will beomitted.

Eighth Embodiment

FIG. 14 shows the construction of a growing apparatus 800 according toan eighth embodiment of the present invention, wherein those partscorresponding to the parts described previously are designated by thesame reference numerals and the description thereof will be omitted.

Referring to FIG. 14, the growing apparatus 800 has a constructionsimilar to that of the growing apparatus 700 of the previous embodiment,except that a cover member 803 is provided so as to cover the freesurface of the melt 102A. Further, the container 601 of a Na—Ga melt iseliminated and a container 801 having a heating mechanism 801A andcontaining therein a molten Na is provided outside the reaction vessel101. Thereby, a vapor of Na is supplied from the container 801 into theinterior of the reaction vessel 101 via a tube 802 and the Na vapor isadded to the atmosphere 107 therein.

According to the present embodiment, uncontrolled precipitation of theGaN fine crystals 102C on the sidewall or bottom surface of the crucible(see FIG. 4B) is minimized, by controlling the vapor pressure of Na fromthe container 801. Further, no GaN precipitation occurs on the melt freesurface, as the free surface of the melt 102A is covered by the covermember 803, except for a central part of the melt where there is formedan opening 803A in the cover member 803 for allowing the seed crystal701 on the rod 702 to make a contact with the surface of the melt 102A.

Thus, according to the construction of FIG. 14, the Na vapor flux actsselectively at the part of the melt 102A where the growth of the bulkGaN ingot is made, and the uncontrolled precipitation of the GaN finecrystals 102C is effectively suppressed.

It should be noted that cover member 803 has a variable geometryconstruction formed of a number of small, fan-shaped members, in whichthe opening 803A can be changed with the growth of the GaN bulk crystal102B in the form of ingot by moving the fan-shaped members in adirection of an arrow Q as represented in FIGS. 15A and 15B, whereinFIG. 15A shows the state in which the central opening 803A of the covermember 803 is closed while FIG. 15B shows the state in which the opening803A has been expanded for allowing the growth of the GaN bulk crystalingot 102B as represented in FIG. 16. It should be noted that FIG. 16shows the growing apparatus 800 in the state that there occurred agrowth of the GaN bulk crystal 102B in the form of ingot.

Ninth Embodiment

FIG. 17 shows the construction of a growing apparatus 900 according to atenth embodiment of the present invention, wherein those partscorresponding to the parts described previously are designated by thesame reference numeral and the description thereof will be omitted.

Referring to FIG. 17, the growing apparatus 900 has a constructionsimilar to that of the growing apparatus 800 of the previous embodiment,except that the tube 802 supplying the Na vapor flux has a sleeve part802A surrounding the rod 702. The sleeve part 802A extends along the rod702 and has an opening 802C in correspondence to the surface of the melt102A where the opening 803A is formed in the cover member 803 for thegrowth of the GaN bulk crystal 102B.

According to the construction of FIG. 17, the Na flux is suppliedselectively to the part where the growth of the GaN bulk crystal 102Btakes place and an efficient growth becomes possible.

As other aspects of the present embodiment are the same as those of theprevious embodiment, further description thereof will be omitted.

Tenth Embodiment

FIG. 18 shows the construction of a growing apparatus 1000 according toa ninth embodiment of the present invention wherein those partscorresponding to the parts described previously are designated by thesame reference numerals and the description thereof will be omitted.

Referring to FIG. 18, the growing apparatus 1000 has a constructionsimilar to that of the growing apparatus 700 of FIG. 12, except that therod 702 driven by the motor 703 and pulling up the seed 701 in theupward direction is replaced by a rod 702′ driven by a motor 703′ andpulls down a seed 701′ in the downward direction. Thus, as representedin FIG. 18, the GaN bulk crystal 102B forms an ingot grown inside themelt 102A. As other aspects of the present invention is the same asthose described before, further description of the present embodimentwill be omitted.

Eleventh Embodiment

In any of the foregoing first through tenth embodiments, the grown ofthe GaN bulk crystal 102B has been achieved at the temperature of650-850° C. under the presence of a Na flux. As mentioned before, theGaN bulk crystal 102B thus obtained has a symmetry of hexagonal crystalsystem.

On the other hand, the inventor of the present invention has discoveredthat a cubic GaN crystal is obtained as the bulk GaN crystal 102Bprovided that the growth is made at a temperature of less than 600° C.under the presence of Na, or when the growth is made at a temperature of650-850° C. under the presence of K. K may be introduced into the systemin the form of a high-purity metallic K starting material, similarly tothe case represented in FIG. 4A.

FIG. 19 shows X-ray diffraction data obtained for a GaN bulk crystalgrown by the apparatus of FIG. 3 as the bulk crystal 102B at atemperature of 750° C. under the total pressure of 7 MPa in the reactionvessel 101. In FIG. 19, it should be noted that the Fo represents thestructural factor obtained from the diffraction pattern for each of thereflections (h k l), while Fc represents the structural factorcalculated from a cubic zinc blende structure. From the diffractionpattern, it was confirmed that the cubic GaN bulk crystal 102B thusformed has a lattice constant a₀ of 4.5062±0.0009 Å. Thus, in thecalculation of the foregoing structural factor Fc, the lattice constanta₀ of 4.5062±0.0009 Å is assumed as the basis of the calculation.Further, FIG. 19 shows an error factor s defined ass=Σ|Fo−Fc|/ΣFb.

Referring to FIG. 19, it can be seen that there is an excellentagreement between the observed structural factor and the calculatedstructural factor assuming the cubic zinc blende structure for theobtained GaN bulk crystal 102B. It can be safely concluded that the GaNbulk crystal 102B obtained in the present embodiment is a 100% cubic GaNcrystal. From the X-ray diffraction analysis, existence of hexagonal GaNcrystal was not detected. Further it was confirmed that the cubic GaNbulk crystal 102B thus formed provides a cathode luminescent peaksubstantially identical with the spectrum of FIG. 5. In other words, thecubic GaN bulk crystal of the present embodiment contains little deepimpurity levels or defects and has an excellent quality characterized bya defect density of 10²-10³ cm⁻² or less.

In view of increasing defect density in the GaN crystals grown at lowtemperatures, and further in view of the fact that a mixture of cubicGaN and hexagonal GaN appears when the growth of the GaN bulk crystal isconducted at the temperature of 600° C. or lower under presence of Naflux, it is preferred to grow a cubic GaN bulk crystal at thetemperature of 650-850° C. under presence of a K flux.

Twelfth Embodiment

FIG. 20 shows the construction of a laser diode 150 of edge-emissiontype according to a twelfth embodiment of the present invention.

Referring to FIG. 20, the laser diode 150 is constructed on a GaN bulkcrystal substrate 151 produced in any of the process explained before.More specifically, the GaN bulk crystal substrate 151 has a high crystalquality characterized by a defect density of 10²-10³ cm⁻² or less.

On the GaN bulk crystal substrate 151, there is provided a lowercladding layer 152 of n-type AlGaN epitaxially with respect to thesubstrate 151 and an optical waveguide layer 153 of n-type GaN is formedon the lower cladding layer 152 epitaxially.

On the optical waveguide layer 153, there is provided an active layer154 of MQW structure including an alternate stacking of quantum welllayers of undoped InGaN having a composition represented asIn_(x)Ga_(1-x)N (x=0.15) and barrier layers of undoped InGaN having acomposition represented as In_(y)Ga_(1-y)N (y=0.02). The active layer154 is covered by an optical waveguide layer 155 of p-type GaN, and anupper cladding layer 156 of p-type AlGaN is formed epitaxially on theoptical waveguide layer 155. Further, a contact layer 157 of p-type GaNis formed on the upper cladding layer 156.

The contact layer 157 and the underlying upper cladding layer 156 aresubjected to a patterning process to form a loss-guide structureextending in the axial direction of the laser diode 150 and theloss-guide structure thus formed is covered by an SiO₂ film 158. TheSiO2 film 158 is formed with an opening 158A extending in the laseraxial direction for exposing the contact layer 157, and a p-typeelectrode 159 is provided on the SiO₂ film 158 in contact with thecontact layer 157 at the opening 158A.

Further, an n-type electrode 160 is provided at a bottom surface of theGaN bulk crystal substrate 151.

After forming the laser structure as such, the layered semiconductorbody including the GaN substrate 151 and the epitaxial layers 151-157 issubjected to a cleaving process to form mirror surfaces M1 and M2defining an optical cavity. Thereby, the laser diode produces a blue toultraviolet optical beam as a result of stimulated emission and opticalamplification occurring in the optical cavity, as represented in FIG. 20by an arrow.

According to the present invention, the optical cavity is formed by asimple cleaving process and the quality of the mirror surfaces M1 and M2defining the optical cavity is improved substantially. Thereby,threshold of laser oscillation is lowered substantially. Further, thelaser diode 150 carries the n-type electrode on the bottom surface ofthe GaN bulk crystal substrate 151 and the process of fabricating thelaser diode is improved substantially. As the epitaxial layers,particularly the GaN optical waveguide layers 153 and 155 and the activelayer 154 sandwiched between the layer 153 and 155 are formedepitaxially on the GaN bulk crystal substrate containing only a verysmall amount of defects, the quality of the crystal constituting theforegoing layers 153-155 is improved substantially over the conventionallaser diode of FIG. 1 and the laser diode 150 of FIG. 20 can be drivenwith a large power. Further, the laser diode 150 of the presentembodiment has an improved lifetime over the conventional laser diode ofFIG. 1.

It should be noted that the GaN bulk crystal substrate 151 may be any ofthe hexagonal type or cubic type. In view of the easiness of cleavingprocess, on the other hand, it is preferable to form the GaN bulkcrystal substrate 151 according to the process of the eleventhembodiment by using a K flux.

Based on the structure of FIG. 20, it is also possible to construct alight-emitting diode. Further, it is possible to construct a verticalcavity laser diode, which produces a laser beam in a direction verticalto the epitaxial layers, also by using the GaN bulk crystal substrate ofthe present invention.

In the case of a vertical cavity laser diode, a pair of mirror surfacesdefining an optical cavity are formed by the epitaxial layers on the GaNbulk crystal substrate 151, and an optical window is formed in theelectrode 159. In such a case, the GaN substrate 151 may have athickness larger than 100 μm such as 300 μm or more.

In the laser diode of FIG. 20, it is also possible to form the mirrorsurfaces M1 and M2 by a dry etching process.

Thirteenth Embodiment

FIG. 21 shows the construction of an electron device 170 constructed ona GaN bulk crystal substrate 171 according to a thirteenth embodiment ofthe present invention.

Referring to FIG. 21, the electron device 170 is an FET, and the GaNbulk crystal 102B of any of the foregoing first through twelfthembodiments is used for the GaN substrate 171.

On the substrate 171, there is provided a high-resistance epitaxiallayer 172 of AlN, and a buffer layer 173 of undoped GaN is formedepitaxially on the AlN high-resistance layer 172.

On the buffer layer 173, a lower barrier layer 174 of undoped AlGaN isformed epitaxially, and a channel layer 175 of undoped GaN is formed onthe lower barrier layer 174 such that the channel layer 175 issandwiched between the lower barrier layer 174 and an upper barrierlayer 176 of undoped AlGaN formed epitaxially on the channel layer 175.

The upper barrier layer 176 is covered by a contact layer 177 of n-typeGaN wherein the layers 174-177 are patterned to form a mesa region fordevice isolation. Further, the contact layer 177 is patterned to exposethe upper barrier layer 176 in correspondence to the channel region, anda Schottky electrode 178 of a Ni/Au structure is provided in contactwith the exposed upper barrier layer 176 as the gate electrode. Further,ohmic electrodes 179 and 180 of a Ti/Al structure are formed on thecontact layer 177 at both lateral sides of the gate electrode 178 as asource electrode and a drain electrode, respectively.

In operation, a two-dimensional electron gas is induced in the channellayer 175 in response to application of a gate voltage to the gateelectrode 178. In this state, the FET is turned on.

According to the present invention, it is thus possible to construct anactive device such as an FET on a GaN substrate, by using the GaN bulkcrystal for the substrate. As the GaN bulk crystal produced according tothe present invention has an high crystal quality characterized by adefect density of 10²-10³ cm⁻² or less, the problem of severe leakagecurrent that would occur when an FET is constructed on a conventionalGaN epitaxial layer formed on a sapphire substrate or an SiC substrate,is successfully eliminated. Further, the construction of FIG. 21 isadvantageous in view of the fact that the electron density of thetwo-dimensional electron gas induced in the channel layer 175 isincreased due to enhanced piezoelectric effect and associated increaseof degree of electron confinement into the channel layer. When thechannel layer contains a high concentration of defects, there occurs alattice relaxation and the effect of carrier confinement is degradedinevitably.

Further, the GaN bulk crystal of the present invention can be used alsoas the GaN substrate of other various electron devices including a HEMT,MESFET and an HBT. In fact, the structure of FIG. 21 can be modified toform a HEMT by employing an n-type AlGaN layer for the upper barrierlayer 176.

Further, the present invention is by no means limited to the embodimentsdescribed heretofore, but various variations and modifications may bemade without departing from the scope of the invention.

1. A crystal having a diameter of 1 cm or more, said crystal showing astrongest peak in cathode luminescent spectrum at a wavelength of 360 nmin correspondence to a band edge.
 2. The crystal as claimed in claim 1,wherein said crystal is free from a peak in cathode luminescent spectrumexcept for said wavelength.
 3. The crystal as claimed in claim 1,wherein said crystal is a cubic system crystal.
 4. The crystal asclaimed in claim 1, wherein said crystal is a GaN crystal.
 5. Thecrystal as claimed in claim 4, wherein said crystal is a GaN singlecrystal.
 6. The crystal as claimed in claim 4, wherein said crystal is aGaN bulk crystal.
 7. The crystal as claimed in claim 6, wherein said GaNbulk crystal belongs to a hexagonal crystal system.
 8. The crystal asclaimed in claim 7, wherein said GaN crystal is a cubic crystal system.9. The crystal as claimed in claim 8, wherein said GaN crystal forms aningot.
 10. The crystal as claimed in claim 4, wherein said GaN crystalforms a slab.
 11. A light-emitting device, comprising: a substrate of aGaN crystal, said GaN crystal having a diameter of 1 cm or more andexhibiting a strongest peak in cathode luminescent spectrum at awavelength of 360 nm in correspondence to a band edge; and an activelayer formed over said substrate.
 12. A transistor, comprising: asubstrate of a GaN crystal, said GaN crystal having a diameter of 1 cmor more and showing a strongest peak in cathode luminescent spectrum ata wavelength of 360 nm in correspondence to a band edge; an insulationlayer formed over said substrate; a buffer layer formed over saidinsulation layer; a first barrier layer formed over said buffer layer; achannel layer formed over said first barrier layer; and second barrierlayer formed over said channel layer, said second barrier layer carryingthereon: a gate electrode; a first contact layer with separation fromsaid gate electrode at a first side of said gate electrode; and a secondcontact layer with separation form said gate electrode at a second,opposite side of said gate electrode, one of said first and secondcontact layers carrying thereon a source electrode, the other of saidfirst and second contact layers carrying thereon a drain electrode. 13.A GaN crystal having a diameter of 1 cm or more, said GaN crystal havinga defect density in the order of of 10²-10³ cm⁻² or less.
 14. The GaNcrystal as claimed in claim 13, wherein said GaN crystal forms a singlecrystal.
 15. The GaN crystal as claimed in claim 13, wherein said GaNcrystal forms a bulk crystal.
 16. The GaN crystal as claimed in claim13, wherein said GaN crystal belongs to a hexagonal crystal system. 17.The GaN crystal as claimed in claim 16, wherein said GaN crystal belongsto a cubic crystal system.
 18. The GaN crystal as claimed in claim 13,wherein said GaN crystal forms an ingot.
 19. A light-emitting device,comprising: a substrate of a GaN crystal, said GaN crystal having adiameter of 1 cm or more and having a defect density in the order of10²-10³ cm⁻² or less; and an active layer formed over said substrate.20. A transistor, comprising: a substrate of a GaN crystal, said GaNcrystal having a diameter of 1 cm or more and having a defect density inthe order of 10²-10³ cm⁻² or less; an insulation layer formed over saidsubstrate; a buffer layer formed over said insulation layer; a firstbarrier layer formed over said buffer layer; a channel layer formed oversaid first barrier layer; and second barrier layer formed over saidchannel layer, said second barrier layer carrying thereon: a gateelectrode; a first contact layer with separation from said gateelectrode at a first side of said gate electrode; and a second contactlayer with separation form said gate electrode at a second, oppositeside of said gate electrode, one of said first and second contact layerscarrying thereon a source electrode, the other of said first and secondcontact layers carrying thereon a drain electrode.